The present invention generally relates to a heat spreader assembly for an electronic package and a method for assembling the heat spreader. More particularly, relates to a three-dimensional stacked heat spreader assembly for holding an electronic device and a method for fabricating the stacked heat spreader assembly.
An important step in the manufacturing of integrated circuits is packaging during which an IC chip is encased in a metal, ceramic or plastic enclosure. A metal package or a ceramic package provides excellent heat dissipation properties, however, they are expensive and labor intensive packaging techniques. For instance, in a ceramic package, the use of two ceramic substrates represents a significant portion of the total cost of fabricating the IC device.
As the production volume for semiconductor devices grew, the development of more cost effective packages becomes increasingly important. One of such cost effective packages developed in recent years is a plastic molded package or sometimes called a plastic quad flat package (PQFP). Although a plastic molded package presents significant fabrication cost advantages, the desirable heat dissipation property that is inherent in a metal or ceramic package is lost. The shortcoming of the poor heat dissipation property in a plastic molded package becomes more severe in the more recently developed IC devices. In modern IC devices, the density of the device has greatly increased. This is because the chip size has become smaller which means that the devices on the chip are being placed closer together. In order to maintain a reasonable service life of an IC device, the operating temperature of the device must be carefully controlled by providing adequate heat dissipation.
Another development in modern IC devices which further demands improved heat dissipation is the increasing use of higher power consumption circuits. For instance, in a conventional 208-pin PQFP device, only 1.0 watt power dissipation is required. The power dissipation, which is closely related to the heat dissipation property becomes more severe in a modern CPU or ASIC chip which requires 2xcx9c3 watts power dissipation capability. The heat dissipation property of a conventional plastic molded package therefore must be improved in order to accommodate the more densely packaged and the higher power consumption IC devices.
A heat sink, normally made of a high thermal conductivity material has been used to fulfill the need for improving heat dissipation in plastic molded packages. The heat sink is sometimes called a heat spreader when a surface of the heat sink is unexposed to the ambient, or called a heat slug when a surface of the heat sink is exposed to the ambient. A heat sink is typically made of a material that has a high thermal conductivity such as copper, copper alloys, aluminum, aluminum alloys or any other high thermal conductivity materials. The heat sink ideally should be in good thermal contact to a semiconductor die.
In a thermal analysis conducted on a ball grid array (BGA) package, it is found that contrary to common misconceptions, most of the heat, i.e., as large as 80% of heat generated by the IC die is transferred downwardly onto a printed circuit board (PCB) by conduction. While only a small percentage, i.e., about 20% is dissipated upwardly by convection and radiation. In order to ensure the reliability or long term service of a ball grid array BGA package, an adequate means must be provided to effectively dissipate heat generated by the IC die during operation of the package.
Various methods for dissipating heat from a BGA package have been proposed by others. For instance, in one of the methods, a heat spreader/heat slug is molded into the plastic molding compound on top of the BGA with a partial surface area of the heat spreader/heat slug exposed. The use of a heat spreader/heat slug combination on top of a BGA package turns out to be ineffective. The reason is that since only 20% of heat generated by the IC die dissipates upwardly, the effectiveness of the heat spreader/heat slug in absorbing and dissipating the heat is limited. Others have used a heat sink of fin type installed on top and in intimate contact with an electronic package. The heat sink while capable of dissipating heat from its multiplicity of pin-shaped fins, the amount of heat actually transferred upwardly into the heat sink is limited. This limits the effectiveness of the heat sink.
It is therefore an object of the present invention to provide an electronic package that is equipped with heat spreaders that does not have the drawbacks or shortcomings of the conventional electronic packages.
It is another object of the present invention to provide an electronic package that is equipped with a three-dimensional stacked heat spreader assembly for the efficient cooling of the package.
It is a further object of the present invention to provide an electronic package that is equipped with an upper heat spreader and at least one lower heat spreader stacked together in a three-dimensional assembly.
In accordance with the present invention, a three-dimensional stacked heat spreader assembly for electronic package and a method for assembling the heat spreader are provided.
In a preferred embodiment, a three-dimensional stacked heat spreader assembly is provided which includes an upper heat spreader of generally rectangular shape having two sides equipped with downwardly extending ridge portions, each of the downwardly extending ridge portions has an inward-facing surface and a. downward-facing surface; a lower heat spreader of generally rectangular shape having two sides equipped with upwardly extending and downwardly extending ridge portions, the upwardly extending ridge portions have an outward-facing surface and an upward-facing surface for engaging the inward-facing surface and the downward-facing surface of the upper heat spreader, respectively., the downwardly extending ridge portions engage a second lower heat spreader that is optionally stacked under the lower heat spreader; and a cavity formed in-between the upper heat spreader and the lower heat spreader adapted for receiving an electronic device.
The three-dimensional stacked heat spreader assembly may further include a heat-conducting material filling a gap formed between the inward-facing surface on the upper heat spreader and the outward-facing surface on the lower heat spreader. The three-dimensional stacked heat spreader assembly may further include a heat conducting material filling a gap between the downward-facing surface on the upper heat spreader and the upward-facing surface on the lower heat spreader. The heat conducting material may be an adhesive or a solder.
The present invention further directs to a three-dimensional electronic package that is encapsulated in stacked heat spreaders which includes an upper heat spreader of generally rectangular shape that has a first edge portion; a lower heat spreader of generally rectangular shape that has a second edge portion for engaging the first edge portion of the upper heat spreader and for forming a cavity therein-between; and an electronic device situated in the cavity between the upper and the lower heat spreaders.
The three-dimensional electronic package encapsulated in stacked heat spreaders may further include a heat-conducting material dispensed in-between the first edge portion and the second edge portion. The heat-conducting material may be an adhesive or a solder. The electronic device may be selected from the group consisting of a ball grid array package, a wire-bonded IC package and a chip scale package. The first edge portion on the upper heat spreader may include a downwardly extending ridge portion that has an inward-facing surface and a downward-facing surface, while the second edge portion on the lower heat spreader may include an outward-facing surface and an upward-facing surface. The lower heat spreader may have a hollow center portion for mounting the electronic device thereover. The three-dimensional electronic package may further include at least a second lower heat spreader that is bonded to a bottom surface of the lower heat spreader forming a second cavity therein-between. adapted for receiving a second electronic device. The electronic device may further include a plurality of solder balls on a bottom surface for establishing electrical communication with a printed circuit board that is positioned under the electronic device.
The present invention further directs to a method for forming a three-dimensional electronic package that is surrounded by stacked heat spreaders including the steps of providing an upper heat spreader of generally rectangular shape that has a first edge portion extending downwardly; providing a lower heat spreader of generally rectangular shape that has a second edge portion extending upwardly; mounting an electronic device on a top surface of the lower heat spreader in such a way that bonding means on the electronic device are exposed through a hollow portion in the lower heat spreader; and sealingly engaging the first edge portion on the upper heat spreader to the second edge portion on the lower heat spreader forming the three-dimensional electronic package.
The method for forming a three-dimensional electronic package may further include the step of sealingly engaging the first edge portion to the second edge portion by using a heat-conducting adhesive or a solder. The method may further include the step of bonding the three-dimensional electronic package to a printed circuit board by bonding means provided on the electronic device. The method may further include the step of selecting the electronic device from the group consisting of a ball grid array package, a wire-bonded IC package and a chip scale package.